376 Prof. S. Lov^n on the Structure of the Echinoidea. 



different from those I liad found in great numbers. He spoke 

 but very little Danish, and it was with difficulty I got to un- 

 derstand from him that he had obtained them from a ship. Going 

 there with him, I saw them unloading sand brought as ballast 

 from Great Britain. It was fortunate I found this out, as I 

 might otherwise have taken them for Icelandic shells from a 

 different part of the island. This is one more instance showing 

 how shells may be transported to countries where they do not 

 occur in a living state, thus causing errors against whicli 

 conchologists cannot be too much on their guard. 



2 Ampton Place, W.O. 



LIII. — On the Structure of the Echinoidea. By S. LoveN. 



[Continued from p. 298.] 



The explanation just given of the development and changes 

 of the ambulacra in the Latistellfe shows that during the growth 

 of the Echinus the primary plates of both rows, as if borne by 

 a slowly flowing stream, are in motion from the point near the 

 eye-plates where they make their appearance, as from its 

 source, down tOAvards the peristome. There the auricles 

 meet, which belong to the masticatory apparatus, not to the 

 corona, with their bases firmly attached to the inside of the 

 oldest plates. It is by their resistance that, in the Latistell^e, 

 the peristome becomes the fixed boundary of the corona towards 

 the buccal membrane, and that, during their growth and 

 the simultaneous downward pressure of the primary plates, 

 the pressure originates of which the consequences are the 

 regular displacement, shifting, and firm coalescence of the 

 plates, which renders the position of the pores apparently 

 confused. 



The Angustistellte, or Cidarida3, present different conditions. 

 In them all the primary plates of the ambulacra are entire 

 plates, continue so always, and distinctly separated from each 

 other by sutures, which are not effaced by any coalescence. 

 They are consequently throughout life like the primary plates 

 in the young of the Latistellse in their first foundation, and 

 form a narrow, single, and uninterrupted sequence, of nearly 

 the same width, which descends gradually in the direction of 

 the margin of the corona, between the margins of the large 

 interradial plates, with regular flexures, which are not original 

 curves, but determined by the margins of the interradial plates. 

 There the bases of the auricles present no resistance ; they 

 remain entirely upon the interradia, by the side of the track of 

 the ambulacra, which tliey leave so open that there is no 



